The subject matter of this disclosure relates generally to power factor correction, and more particularly, to a system and method for compensating for the leading current in an electrical load such as an LED driver input during light loading of the LED driver.
Power factor correctors (PFCs) are well known. Boost converters, Flyback converters, and other topologies are generally used to provide power factor correction for devices powered from an AC line. Input capacitors are typically placed both before and after the rectifier in a boost or other PFC converter to control electro-magnetic interference (EMI). The input capacitors adversely affect power factor (PF).
FIG. 1 is a schematic diagram illustrating a typical power factor corrector 10 that includes EMI reduction capacitors C1 and C2. A current regulator IPFC regulates the PFC converter current at the input to an LED driver, not shown. The LED driver input current IIN=(IPFC+C1ωVm cos ωt)·sign(Vin(t))+C2ωVm cos(ωt), where IPFC is regulated to a sinusoid in most cases. However, at high line voltages and light loading conditions, the capacitor currents dominate, causing poor PF.
FIG. 2 is a graph illustrating several waveforms, including instant power 18, ideal PFC current or reference current signal 20 after a diode rectifier, converter input current 22, power factor capacitor current 24 and the input voltage 26. The waveforms shown correspond to regulated current waveforms that are typical under light loading conditions. The net input current is the sum and leads the PFC converter applied voltage. The current in capacitors C1 and C2 for example, causes the input current shape to not align with the input voltage at light loads, and becomes worse at high line.
Known methods of achieving good PF at the input to an LED driver are limited in both scope and application. Applicable methods usually involve sensing the capacitor current and closing the loop to control the average input current with a feedback loop. This method however, does not translate to other known methods of power factor correction, such as a boundary conduction mode boost or a discontinuous mode Flyback converter. A need exists therefore, for a technique of controlling PF at the input to an LED driver that is not limited in both scope and application, such that the technique can be applied to a broader class of control methods.